The present invention relates to washers for threaded fasteners and to assemblies of washers and threaded fasteners that are particularly useful for mounting wheels to motor vehicles such as trucks.
Numerous special problems are encountered in the wide range of application of male and female threaded fasteners. These problems include the inability of the threaded fastener to deflect under conditions of varying load, loosening during operation, hoop-spreading due to overtightening or overloading and brinelling into the surface of the wheel. In motor vehicle wheel applications, the maintenance of the installed torque under conditions of varying load during use and the loosening of nuts following initial assembly are of particular concern. In case of the latter, loosening is believed to be associated with the tendency of new parts to seat themselves and the thinning of paint layers due to further drying.
The foregoing problems have been solved to a degree by the washer disclosed in U.S. Pat. No. 4,971,498, owned by the assignee of this application; the disclosure of the patent being incorporated herein by reference. The washer of the aforesaid patent provides a cantilever body portion that may be selectively sized to control the degree of deflection resulting upon loading of the washer, but this approach has not been entirely satisfactory or structurally efficient in all cases.
Other examples of solutions to the problem of the inability to deflect under varying load conditions include the following. U.S. Pat. No. 1,352,918 discloses a pair of seat plates which are each provided with an annular flange creating a recessed cavity on its inner face. The seat plates further have a concavely-shaped seat for receiving a convexly-shaped portion of a bolt or nut. U.S. Pat. No. 1,961,470 discloses a frusto-spherical cup-shaped washer which is rotatably connected to a nut at a conically-shaped seat. Each of these solutions suffer from the drawback that they impose columnar loading directly onto the mating surface when a force is imposed which is along the axis of the threaded fastener. As a result, the variation of loading conditions is accommodated by compression, the degree of which depends upon the modulus of the material. Compressive response to variations in loading conditions is not a true deflection response, and usually results in brinelling into the mating surface, as well as fatigue failure and unpredictable loosening of the threaded fastener.